It is known from the prior art it is known how to use for protection against motor heat overload, in addition to an overload relay which recognizes an overloading of the motor with the aid of the motor's current uptake, also thermistor protective circuits, for example, which detect and evaluate the temperature of a motor winding. Various temperature detector elements are used for detecting the temperature of the motor winding, such as temperature sensors, especially PTC (PTC: positive temperature coefficient) thermistors, PTIOO sensors and KTY sensors (with PTC characteristic), whose electrical resistance changes in dependence on the detected temperature. The mode of operation of the overtemperature protection from the solutions known in the prior art with a PTC sensor is based on the principle that two temperature-dependent resistance regions are used. The resistance of such a temperature detector element lies in a first resistance region for as long as the motor winding is in a temperature range which presents no problems. The second resistance region, whose resistance values are for example larger than those of the first resistance region, then corresponds to a temperature above a system-specific threshold value.
Furthermore, so-called thermo-click elements, also known as temperature monitors, can also be used as temperature detector elements. These generally constitute simple switches, usually formed by means of a bimetal, which is closed in the first temperature region and which opens in a second temperature region, i.e., above the temperature threshold value. That is, the resistance changes from almost 0Ω to infinite resistance.
In the first mentioned temperature detector elements, the temperature sensors, the overloading of the motor is assessed by the resistance change in the temperature detector elements within predefined value ranges. On the other hand, the thermo-click element in its customary area of application cuts out the current through a motor winding upon reaching the temperature threshold value. Temperature sensors, on the contrary, are assessed by means of a protective circuit, which basically determines the electrical resistance of the particular temperature detector elements, and puts out a message signal to a switch, an evaluation circuit, or the like, thereby triggering e.g. a shut-off of the device or starting some other predetermined function if a thermal overload is recognized depending on the measured temperature. At rather large winding currents, the sensor element or the temperature monitor does not directly switch the winding current, but instead acts on the power supply or the actuating of the power end stage.
From EP 2535993 A1 there is known an actuating circuit for an electronically commutated, collector-less d.c. motor, i.e., a so-called EC motor with a semiconductor end stage at a d.c. power supply potential, which is actuated by an electronic commutation control system across a driver stage for the time-offset actuation of stator windings of the motor in order to generate a rotating magnetic field for a rotor in dependence on the rotor's rotary position.
Such EC motors, in keeping with the present national and international standards, need to be protected against overheating in event of abnormal operating situations.
It is furthermore known how to design impedance-protected motors, i.e., how to dimension the winding resistance and the wire diameter so that no overheating can occur during abnormal operation. But a design with impedance protection is not possible for EC motors, especially those with single-tooth developments, because the winding resistances are very small.
Therefore, so far a software current limitation has been integrated in the commutation control system, which assigns a lower threshold value for the starting current and an upper threshold value for the operating current. If the lower threshold value is reached or surpassed in the starting phase, the motor will be switched off. But if the motor starts correctly, it switches to a monitoring of the operating current in regard to the upper threshold value, and if the upper threshold value is reached or surpassed the motor will be switched off.
However, such protective measures realized by software alone are not taken into consideration by many authorization entities, such as UL (Underwriters Laboratories), during the authorization testing for compliance with a valid UL standard in order to award the required approval.
Moreover, there are problems which need to be considered in practice on account of a rapid temperature rise. Thus, e.g., in the case of a rotor blockage of the motor, rapid rises in winding temperature may occur depending on the winding design. As a result, unacceptably high winding temperatures occur, but they do not bring about a timely shut-off of the winding current on account of the sluggish temperature monitors, whose response is too slow.